add pre-commit configuration

This enables use of the `pre-commit` framework to run quick pre-commit
checks. In particular this allows to automatically fix:
* trailing white spaces
* absence or multiple newlines at the end of files
* QL code formatting
* file sync

More could be added in the future: anything that can be checked fast
can be added in the configuration (for example well-formedness of
`qldoc` files).

This is a purely opt-in feature. Instructions for enabling it and
possibly configuring its behaviour are in `pre-commit-hook-setup.md`.

.pre-commit-config.yaml

@@ -0,0 +1,21 @@
+# See https://pre-commit.com for more information
+# See https://pre-commit.com/hooks.html for more hooks
+exclude: /test/.*$(?<!\.ql)(?<!\.qll)(?<!\.qlref)
+repos:
+-   repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v3.2.0
+    hooks:
+    -   id: trailing-whitespace
+    -   id: end-of-file-fixer
+-   repo: local
+    hooks:
+    -   id: codeql-format
+        name: Fix QL file formatting
+        files: \.qll?$
+        language: system
+        entry: codeql query format --in-place
+    -   id: sync-files
+        name: Fix files required to be identical
+        language: system
+        entry: python3 config/sync-files.py --latest
+        pass_filenames: false

CONTRIBUTING.md

@@ -42,7 +42,11 @@ If you have an idea for a query that you would like to share with other CodeQL u
 
     - The queries and libraries must be autoformatted, for example using the "Format Document" command in [CodeQL for Visual Studio Code](https://help.semmle.com/codeql/codeql-for-vscode/procedures/about-codeql-for-vscode.html).
 
-    If you prefer, you can use this [pre-commit hook](misc/scripts/pre-commit) that automatically checks whether your files are correctly formatted. See the [pre-commit hook installation guide](docs/pre-commit-hook-setup.md) for instructions on how to install the hook.
+    If you prefer, you can either:
+    1. install the [pre-commit framework](https://pre-commit.com/) and install the configured hooks on this repo via `pre-commit install`, or
+    2. use this [pre-commit hook](misc/scripts/pre-commit) that automatically checks whether your files are correctly formatted.
+
+    See the [pre-commit hook installation guide](docs/pre-commit-hook-setup.md) for instructions on the two approaches.
 
 4. **Compilation**
 

docs/pre-commit-hook-setup.md

@@ -14,3 +14,21 @@ ql/cpp/ql/src/printAst.ql would change by autoformatting.
 ```
 
 If you prefer to have the script automatically format the code (and not abort the commit), you can replace the line `codeql query format --check-only` with `codeql query format --in-place` (and `exit $exitVal` with `exit 0`).
+
+## Using the `pre-commit` framework
+
+Alternatively, you can use the [pre-commit framework](https://pre-commit.com/). There are some pre-commit hooks already configured on [`.pre-commit-config.yaml`](../.pre-commit-config.yaml). In order to install them you need to follow pre-commit's [installation instructions](https://pre-commit.com/#installation) and then run `pre-commit install`.
+
+By default, pre-commit will check and fix
+* trailing whitespaces;
+* absence of double newlines at end of files;
+* QL formatting;
+* files out of sync (see [`config/sync-files.py`](../config/sync-files.py)).
+
+It will run the checks only on files changed by the commit (except for the file sync check) and it will skip all files under `test` directories unless they are `.ql`, `.qll` or `.qlref` files.
+
+If you want to change one of these default behaviours (for example, you want to skip the out-of-sync file check, or you prefer to pass `--check-only` instead of `--in-place` to the query formatter), run
+```
+git update-index --assume-unchanged .pre-commit-config.yaml
+```
+and you can then modify the configuration at your will.